An hour, or a couple of hours - it's really not necessary if they are out of the elements and don't have any draft blowing directly on them. The point Aart was trying to make is, they have their nice warm down coats on and are acclimated to the cooler weather. Bringing them in is probably more uncomfortable for them than remaining in consistent temperatures.@aart To answer your snarky question and criticism, I brought the hens into the house when there was snow on the ground and temperatures were so cold that their water was freezing within about an hour. They were in a part of the house that stays about 60 degrees in the winter. They seemed to enjoy it and my laying hen has never missed a day without laying an egg, and they're all four healthy birds, so I don't think warming them up for an hour was particularly unhealthy.
If you're worried about their water freezing, you could bring them warm water a few times a day since you're home anyway. Or, if you frequently get freezing water, a heated dog dish isn't that expensive.